Separation of citric acid from fermentation broth with a weakly basic anionic exchange resin adsorbent

ABSTRACT

Citric acid is separated from a fermentation broth by using an adsorbent comprising a water-insoluble macroreticular or gel weakly basic anionic exchange resin possessing tertiary amine functional groups or pyridine functional groups, said anionic exchange resin comprising a cross-linked acrylic or styrene resin matrix. Citric acid is desorbed by water or dilute sulfuric acid. The pH of the feed is maintained below the first ionization constant (pKa 1 ) of citric acid to maintain selectivity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation in part of Ser. No. 943,219, filed Dec. 18, 1986.A related case, filed concurrently herewith, Ser. No. Ser. No. 122,161,filed Nov. 16, 1987 is also a continuation-in-part of said Ser. No.943,219.

FIELD OF THE INVENTION

The field of art to which this invention pertains is the solid bedadsorptive separation of citric acid from fermentation broths containingcitric acid, carbohydrates, amino acids, proteins and salts. Morespecifically, the invention relates to a process for separating citricacid which process employs an adsorbent comprising particular polymerswhich selectively adsorb citric acid from a fermentation mixturecontaining citric acid.

BACKGROUND OF THE INVENTION

Citric acid is used as a food acidulant, and in pharmaceutical,industrial and detergent formulations. The increased popularity ofliquid detergents formulated with citric acid has been primarilyresponsible for growth of worldwide production of citric acid to about700 million pounds per year which is expected to continue in the future.

Citric acid is produced by a submerged culture fermentation processwhich employs molasses as feed and the microorganism, Aspergillus Niger.The fermentation product will contain carbohydrates, amino acids,proteins and salts as well as citric acid, which must be separated fromthe fermentation broth.

There are two technologies currently employed for the separation ofcitric acid from fermentation broths containing the same. The firstinvolves calcium salt precipitation of citric acid. The resultingcalcium citrate is acidified with sulfuric acid. In the second process,citric acid is extracted from the fermentation broth with a mixture oftrilauryl-amine, n-octanol and a C₁₀ or C₁₁ isoparaffin. Citric acid isreextracted from the solvent phase into water with the addition of heat.Both techniques, however, are complex, expensive and they generate asubstantial amount of waste for disposal.

The patent literature has suggested a possible third method forseparating citric acid from the fermentation broth, which involvesmembrane filtration to remove raw materials or high molecular weightimpurities and then adsorption of contaminants onto a nonionic resinbased on polystyrene or polyacrylic resins and collection of the citricacid in the rejected phase or raffinate and crystallization of thecitric acid after concentrating the solution, or by precipitating thecitric acid as the calcium salts then acidifying with H₂ SO₄, separatingthe CaSO₄ and contacting cation- and anion-exchangers. This method,disclosed in European Published Application No. 151,470, Aug. 14, 1985,is also a rather complex and lengthy method for separating the citricacid. In contrast, our method makes it possible to separate the citricacid in a single step and recover the citric acid in a much simplifiedprocess. Succinctly stated, the citric acid is adsorbed selectively bythe adsorbent and purified citric acid is desorbed by a desorbent, forexample, water or a dilute acid, e.g., sulfuric acid, hydrochloric acid,nitric acid or phosphoric acid.

The establishment of pH below the pK value in an adsorbent separation ofcitric acid from other acids is disclosed in U.S. Pat. No. 2,664,441,but there is no suggestion of or rationale for the application thereofto the instant separation of citric acid from mixtures thereof withnon-acidic components.

SUMMARY OF THE INVENTION

This invention relates to a process for adsorbing citric acid from afermentation broth onto a weakly basic, macroreticular or gel type,water-insoluble, anionic exchange resin matrix possessing tertiary amineor pyridine functional groups. The resin matrix is either acrylic orstyrene, cross-linked with divinylbenzene. The citric acid is recoveredby desorption with a water or a dilute inorganic acid, especially,sulfuric acid, desorbent under desorption conditions. Concentrations ofinorganic acid of about 0.01N to about 1.0N can be used in theinvention, preferably 0.1 to 0.2N. These resins result in an improvedseparation over the neutral resins disclosed in the parent applicationabove referred to. They are superior in the adsorption separation ofcitric acid disclosed therein in their increased stability todeactivation by impurities in the feed. While these resins are known tobe chemically and physically stable and useful absorbents, their utilityfor the separation of citric acid has never been described publicly. Toour knowledge, the first disclosure of this class of (resin) adsorbentsto separate citric acid from a fermentation broth was made in our parentcase, above referred to. One aspect of the invention is in the discoverythat complete separation of citric acid from salts and carbohydrates isonly achieved by adjusting and maintaining the pH of the feed solutionlower than the first ionization constant (pKa₁) of citric acid (3.13).However, pH's in the range of 0.5 to 2.5 are preferred and 1.5 to 2.2are more preferred.

The invention also relates to a process for separating citric acid froma feed mixture comprising a fermentation broth, which process employs awater-insoluble, macroreticular or gel, weakly basic anionic exchangeresin possessing tertiary amine functional groups or pyridine functionalgroups, said anionic exchange resin having a cross-linked acrylic orstyrene resin matrix which comprises the steps of:

(a) maintaining net fluid flow through a colume of said adsorbent in asingle direction, which column contains at least three zones havingseparate operational functions occurring therein and being seriallyinterconnected with the terminal zones of said column connected toprovide a continuous connection of said zones;

(b) maintaining an adsorption zone in said column, said zone defined bythe adsorbent located between a feed input stream at an upstreamboundary of said zone and a raffinate output stream at a downstreamboundary of said zone;

(c) maintaining a purification zone immediately upstream from saidadsorption zone, said purification zone defined by the absorbent locatedbetween an extract output stream at an upstream boundary of saidpurification zone and said feed input stream at a downstream boundary ofsaid purification zone;

(d) maintaining a desorption zone immediately upstream from saidpurification zone, said desorption zone defined by the adsorbent locatedbetween a desorbent input stream at an upstream boundary of said zoneand said extract output stream at a downstream boundary of said zone;

(e) passing said feed mixture into said adsorption zone at adsorptionconditions to effect the selective adsorption of said citric acid bysaid adsorbent in said adsorption zone and withdrawing a raffinateoutput stream comprising the nonadsorbed components of said fermentationbroth from said adsorption zone;

(f) passing a desorbent material into said desorption zone at desorptionconditions to effect the displacement of said citric acid from theadsorbent in said desorption zone;

(g) withdrawing an extract output stream comprising said citric acid anddesorbent material from said desorption zone;

(h) passing at least a portion of said extract output stream to aseparation means and therein separating at separation conditions atleast a portion of said desorbent material; and,

(i) periodically advancing through said column of adsorbent in adownstream direction with respect to fluid flow in said adsorption zonethe feed input stream, raffinate output stream, desorbent input stream,and extract output stream to effect the shifting of zones through saidadsorbent and the production of extract output and raffinate outputstreams. At least a portion of said raffinate stream may be passed to aseparation means at separation conditions, thereby separating at least aportion of said desorbent material, to produce a raffinate producthaving a reduced concentration of desorbent material. Further, a bufferzone may be maintained immediately upstream from said desorption zone,said buffer zone defined as the adsorbent located between the desorbentinput stream at a downstream boundary of said buffer zone and theraffinate output stream at an upstream boundary of said buffer zone.

Other aspects of the invention encompass details of feed mixtures,adsorbents, desorbents and operating conditions which are hereinafterdisclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of concentration of various citric acid species versusthe pH of citric acid dissociation which shows the shifting of theequilibrium point of the citric acid dissociation by varying theconcentration of citric acid, citrate anions and the hydrogen ion.

FIG. 2 is a static plot that shows the increase in the amount of citricacid absorbed by the neutral absorbent, XAD-4, of the parent case, asthe pH is lowered, confirming the equilibrium shift set forth in thepreceding paragraph and the parent case.

FIGS. 3-6 are the plots of the pulse tests in Example I using a weaklybasic anionic exchange resin having a tertiary amine functionality in across-linked acrylic resin matrix to separate citric acid from a feedcontaining 40% citric acid at a pH of 1.6, desorbed with water, 0.05N,0.25N, and 0.05N sulfuric acid. The excellent stabiity of this adsorbentcan be seen by a comparison of FIGS. 4 and 6, wherein FIG. 6 shows noloss of separation capability after 24 bed volumes of fermentation brothhave been passed through the adsorbent.

FIGS. 7A, 7B and 7C are plots of the pulse tests of Example II, at pHsof 7.0, 3.5 and 2.4, respectively.

FIGS. 8, 9A and 9B are the plots of the pulse test of Example III at apH of 1.6 run on several different adsorbent samples of weakly basicanionic exchange resin possessing pyridine functionality in across-linked polystyrene resin matrix. The citric acid is desorbed with0.05N sulfuric acid or water.

FIGS. 10, 11A, 11B and 12 are plots of the pulse tests of Example IV, atpHs of 1.6, 2.2, 2.2 and 1.6, respectively, in which another weaklybasic anionic exchange resins possessing tertiary amine functionality ina cross-linked acryic resin matrix are used. The desorbents at 0.05N and0.15N H₂ SO₄.

DESCRIPTION OF THE INVENTION

At the outset the definitions of various terms used throughout thespecification will be useful in making clear the operation, objects andadvantages of our process.

A "feed mixture" is a mixture containing one or more extract componentsand one or more raffinate components to be separated by our process. Theterm "feed stream" indicates a stream of a feed mixture which passes tothe adsorbent used in the process.

An "extract component" is a compound or type of compound that is moreselectively adsorbed by the adsorbent while a "raffinate component" is acompound or type of compound that is less selectively adsorbed. In thisprocess, citric acid is an extract component and proteins, amino acids,salts and carbohydrates are raffinate components. The term "desorbentmaterial" shall mean generally a material capable of desorbing anextract component. The term "desorbent stream" or "desorbent inputstream" indicates the stream through which desorbent material passes tothe adsorbent. The term "raffinate stream" or "raffinate output stream"means a stream through which a raffinate component is removed from theadsorbent. The composition of the raffinate stream can vary fromessentially 100% desorbent material to essentially 100% raffinatecomponents. The term "extract stream" or "extract output stream" shallmean a stream through which an extract material which has been desorbedby a desorbent material is removed from the adsorbent. The compositionof the extract stream, likewise, can vary from essentially 100%desorbent material to essentially 100 % extract components. At least aportion of the extract stream and preferably at least a portion of theraffinate stream from the separation process are passed to separationmeans, typically fractionators, where at least a portion of desorbentmaterial is separated to produce an extract product and a raffinateproduct. The terms "extract product" and "raffinate product" meanproducts produced by the process containing, respectively, an extractcomponent and a raffinate component in higher concentrations than thosefound in the extract stream and the raffinate stream. Although it ispossible by the process of this invention to produce a high purity,citric acid product at high recoveries, it will be appreciated that anextract component is never completely adsorbed by the adsorbent.Likewise, a raffinate component is completely nonadsorbed or onlyslightly adsorbed by the adsorbent. Therefore, varying amounts of araffinate component can appear in the extract stream and, likewise,varying amounts of an extract component can appear in the raffinatestream. The extract and raffinate streams then are further distinguishedfrom each other and from the feed mixture by the ratio of theconcentrations of an extract component and a raffinate componentappearing in the particular stream. More specifically, the ratio of theconcentration of citric acid to that of the less selectively adsorbedcomponents will be lowest in the raffinate stream, next highest in thefeed mixture, and the highest in the extract stream. Likewise, the ratioof the concentration of the less selectively adsorbed components to thatof the more selectively adsorbed citric acid will be highest in theraffinate stream, next highest in the feed mixture, and the lowest inthe extract stream.

The term "selective pore volume" of the adsorbent is defined as thevolume of the adsorbent which selectively adsorbs an extract componentfrom the feed mixture. The term "nonselective void volume" of theadsorbent is the volume of the adsorbent which does not selectivelyretain an extract component from the feed mixture. This volume includesthe cavities of the adsorbent which contain no adsorptive sites and theinterstitial void spaces between adsorbent particles. The selective porevolume and the nonselective void volume are generally expressed involumetric quantities and are of importance in determining theproperflow rates of fluid required to be passed into an operational zonefor efficient operations to take place for a given quantity ofadsorbent. When adsorbent "passes" into an operational zone (hereinafterdefined and described) employed in one embodiment of this process itsnonselective void volume together with its selective pore volume carriesfluid into that zone. The nonselective void volume is utilized indetermining the amount of fluid which should pass into the same zone ina countercurrent direction to the adsorbent to displace the fluidpresent in the nonselective void volume. If the fluid flow rate passinginto a zone is smaller than the nonselective void volume rate ofadsorbent material passing into that zone, there is a net entrainment ofliquid into the zone by the adsorbent. Since this net entrainment is afluid present in nonselective void volume of the adsorbent, it in mostinstances comprises less selectively retained feed components. Theselective pore volume of an adsorbent can in certain instances adsorbportions of raffinate material from the fluid surrounding the adsorbentsince in certain instances there is competition between extract materialand raffinate material for adsorptive sites within the selective porevolume. If a large quantity of raffinate material with respect toextract material surrounds the adsorbent, raffinate material can becompetitive enough to be adsorbed by the adsorbent.

The feed material contemplated in this invention is the fermentationproduct obtained from the submerged culture fermentation of molasses bythe microorganism, Aspergillus Niger. The fermentation product will havea composition exemplified by the following:

    ______________________________________                                        Citric acid             12.9%                                                 Salts                   6,000 ppm                                             Carbohydrates (sugars)  1%                                                    Others (proteins and amino acids)                                                                     2%                                                    ______________________________________                                    

The salts will be K, Na, Ca, Mg and Fe. The carbohydrates are sugarsincluding glucose, xylose, mannose, oligosaccharides of DP2 and DP3 plusas many as 12 or more unidentified saccharides. The composition of thefeedstock may vary from that given above and still be used in theinvention. However, juices such as citrus fruit juices, are notacceptable or contemplated because other materials contained thereinwill be adsorbed at the same time rather than citric acid alone.Johnson, J. Sci. Food Agric., Vol 33 (3) pp 287-93.

I have discovered that the separation of citric acid can be enhancedsignificantly by adjusting the pH of the feed to a level below the firstionization constant of citric acid. The first ionization constant (pKa₁)of citric acid is 3.13, Handbook of Chemistry & Physics, 53rd Edition,1972-3, CRC Press, and therefore, the pH of the citric acid feed shouldbe below 3.13. When the pH for a 40% concentrated solution of citricacid is 3.5 or greater, for example, as in FIGS. 7A and 7B (Example II)citric acid "breaks through" (is eluted) with the salts andcarbohydrates at the beginning of the cycle, indicating that most of thecitric acid is not adsorbed. In contrast, no "break through" of citricacid is observed when the pH is below 2.2 for example as in FIGS. 11Aand 11B (Example IV), respectively. We cannot state the reasons for thiseffect in this case, but, without being bound by our theory, we believethat the explanation given in the parent case may be correct, and isincorporated herein in its entirety by reference. Briefly stated, asdemonstrated in FIGS. 1 and 2 herein, the concentration of nonionizedcitric acid (the adsorbed species) is increased, while reducing thecitrate species (H₂ CA⁻¹, HCA⁻² and CA⁻³) in the solution, as the pH isdecreased.

Further, looking at both the tertiary amine- andpyridine-function-containing ion exchange resins of the presentinvention, the lone pair electron from the nitrogen atom can hydrogenbond to the citric acid either directly or through a sulfate ion, as forexample, with a tertiary amine: ##STR1## and with a pyridinefunction-containing resin: ##STR2##

At higher pH (3.1) feed, there will be insufficient hydrogen ions forthe hydrogen bond formation with the amine nitrogen or the sulfate ion;citric acid will not be adsorbed by the resin and will "break through"with salts and carbohydrates at the beginning of the cycle.

A similar explanation may be envisioned for pyridine function-containingresins.

Desorbent materials used in various prior art adsorptive separationprocesses vary depending upon such factors as the type of operationemployed. In the swing bed system, in which the selectively adsorbedfeed component is removed from the adsorbent by a purge stream,desorbent selection is not as critical and desorbent materialscomprising gaseous hydrocarbons such as methane, ethane, etc., or othertypes of gases such as nitrogen or hydrogen may be used at elevatedtemperatures or reduced pressures or both to effectivey purge theadsorbed feed component from the adsorbent. However, in adsorptiveseparation processes which are generally operated continuously atsubstantially constant pressures and temperatures to insure liquidphase, the desorbent material must be judiciously selected to satisfymany criteria. First, the desorbent material should displace an extractcomponent from the adsorbent with reasonable mass flow rates withoutitself being so strongly adsorbed as to unduly prevent an extractcomponent from displacing the desorbent material in a followingadsorption cycle. Expressed in terms of the selectivity (hereinafterdiscussed in more detail), it is preferred that the adsorbent be moreselective for all of the extract components with respect to a raffinatecomponent than it is for the desorbent material with respect to araffinate component. Secondly, desorbent materials must be compatiblewith the particular adsorbent and the particular feed mixture. Morespecifically, they must not reduce or destroy the critical selectivityof the adsorbent for an extract component with respect to a raffinatecomponent. Desorbent materials should additionally be substances whichare easily separable from the feed mixture that is passed into theprocess. Both the raffinate stream and the extract stream are removedfrom the adsorbent in admixture with desorbent material and without amethod of separating at least a portion of the desorbent material thepurity of the extract product and the raffinate product would not bevery high, nor would the desorbent material be available for reuse inthe process. It is therefore contemplated that any desorbent materialused in this process will preferably have a substantially differentaverage boiling point than that of the feed mixture to allow separationof at least a portion of the desorbent material from feed components inthe extract and raffinate streams by simple fractional distillationthereby permitting reuse of desorbent material in the process. The term"substantially different" as used herein shall mean that the differencebetween the average boiling points between the desorbent material andthe feed mixture shall be at least about 5° C. The boiling range of thedesorbent material may be higher or lower than that of the feed mixture.Finally, desorbent materials should also be materials which are readilyavailable and therefore reasonable in cost. In the preferred isothermal,isobaric, liquid phase operation of the process of our invention, wehave found dilute sulfuric acid (0.01 to 1.0N) a particularly effectivedesorbent material. Also, other dilute inorganic acids, such ashydrochloric acid, nitric acid, phosphoric acid and water may be used asa desorbent, but they will be found to be less effective.

The prior art has also recognized that certain characteristics ofadsorbents are highly desirable, if not absolutely necessary, to thesuccessful operation of a selective adsorption process. Suchcharacteristics are equally important to this process. Among suchcharacteristics are: (1) adsorptive capacity for some volume of anextract component per volume of adsorbent; (2) the selective adsorptionof an extract component with respect to a raffinate component and thedesorbent material; and (3) sufficiently fast rates of adsorption anddesorption of an extract component to and from the adsorbent. Capacityof the adsorbent for adsorbing a specific volume of an extract componentis, of course, a necessity; without such capacity the adsorbent isuseless for adsorptive separation. Furthermore, the higher theadsorbent's capacity for an extract component the better is theadsorbent. Increased capacity of a particular adsorbent makes itpossible to reduce the amount of adsorbent needed to separate an extractcomponent of known concentration contained in a particular charge rateof feed mixture. A reduction in the amount of adsorbent required for aspecific adsorptive separation reduces the cost of the separationprocess. It is important that the good initial capacity of the adsorbentbe maintained during actual use in the separation process over someeconomically desirable life. The second necessary adsorbentcharacteristic is the ability of the adsorbent to separate components ofthe feed; or, in other words, that the adsorbent possess adsorptiveselectivity, (β), for one component as compared to another component.Relative selectivity can be expressed not only for one feed component ascompared to another but can also be expressed between any feed mixturecomponent and the desorbent material. The selectivity, (β), as usedthroughout this specification is defined as the ratio of the twocomponents of the adsorbed phase over the ratio of the same twocomponents in the unadsorbed phase at equilibrium conditions. Relativeselectivity is shown as Equation 1 below: ##EQU1## where C and D are twocomponents of the feed represented in volume percent and the subscriptsA and U represent the adsorbed and unadsorbed phases respectively. Theequilibrium conditions were determined when the feed passing over a bedof adsorbent did not change composition after contacting the bed ofadsorbent. In other words, there was no net transfer of materialoccurring between the unadsorbed and adsorbed phases. Where selectivityof two components approaches 1.0 there is no preferential adsorption ofone component by the adsorbent with respect to the other; they are bothadsorbed (or nonadsorbed) to about the same degree with respect to eachother. As the (β) becomes less than or greater than 1.0 there is apreferential adsorption by the adsorbent for one component with respectto the other. When comparing the selectivity by the adsorbent of onecomponent C over component D, a (β) larger than 1.0 indicatespreferential adsorption of component C within the adsorbent. A (β) lessthan 1.0 would indicate that component D is preferentially adsorbedleaving an unadsorbed phase richer in component C and an adsorbed phasericher in component D. Ideally desorbent materials should have aselectivity equal to about 1 or slightly less than 1 with respect to allextract components so that all of the extract components can be desorbedas a class with reasonable flow rates of desorbent material and so thatextract components can displace desorbent material in a subsequentadsorption step. While separation of an extract component from araffinate component is theoretically possible when the selectivity ofthe adsorbent for the extract component with respect to the raffinatecomponent is greater than 1, it is preferred that such selectivityapproach a value of 2. Like relative volatility, the higher theselectivity, the easier the separation is to perform. Higherselectivities permit a smaller amount of adsorbent to be used. The thirdimportant characteristic is the rate of exchange of the extractcomponent of the feed mixture material or, in other words, the relativerate of desorption of the extract component. This characteristic relatesdirectly to the amount of desorbent material that must be employed inthe process to recover the extract component from the adsorbent; fasterrates of exchange reduce the amount of desorbent material needed toremove the extract component and therefore permit a reduction in theoperating cost of the process. With faster rates of exchange, lessdesorbent material has to be pumped through the process and separatedfrom the extract stream for reuse in the process.

A dynamic testing apparatus is employed to test various adsorbents witha particular feed mixture and desorbent material to measure theadsorbent characteristics of adsorptive capacity, selectivity andexchange rate. The apparatus consists of an adsorbent chamber comprisinga straight or helical column of approximately 70 cc volume having inletand outlet portions at opposite ends of the chamber. The chamber iscontained within a temperature control means and, in addition, pressurecontrol equipment is used to operate the chamber at a constantpredetermined pressure. Quantitative and qualitative analyticalequipment such as refractometers, polarimeters and chromatographs can beattached to the outlet line of the chamber and used to detectquantitatively or determine qualitatively one or more components in theeffluent stream leaving the adsorbent chamber. A pulse test, performedusing this apparatus and the following general procedure, is used todetermine selectivities and other data for various adsorbent systems.The adsorbent is filled to equilibrium with a particular desorbentmaterial by passing the desorbent material through the adsorbentchamber. At a convenient time, a pulse of feed containing knownconcentrations of a tracer and of a particular extract component or of araffinate component or both, all diluted in desorbent, is injected for aduration of several minutes. Desorbent flow is resumed, and the tracerand the extract component or the raffinate component (or both) areeluted as in a liquid-solid chromatographic operation. The effluent canbe analyzed onstream or, alternatively, effluent samples can becollected periodically and later analyzed separately by analyticalequipment and traces of the envelopes of corresponding component peaksdeveloped.

From information derived from the test adsorbent, performance can be interms of void volume, retention volume for an extract or a raffinatecomponent, selectivity for one component with respect to the other, andthe rate of desorption of an extract component by the desorbent. Theretention volume of an extract or a raffinate component may becharacterized by the distance between the center of the peak envelope ofan extract or a raffinate component and the peak envelope of the tracercomponent or some other known reference point. It is expressed in termsof the volume in cubic centimeters of desorbent pumped during this timeinterval represented by the distance between the peak envelopes.Selectivity, (β), for an extract component with respect to a raffinatecomponent may be characterized by the ratio of the distance between thecenter of the extract component peak envelope and the tracer peakenvelope (or other reference point) to the corresponding distancebetween the center of the raffinate component peak envelope and thetracer peak envelope. The rate of exchange of an extract component withthe desorbent can generally be characterized by the width of the peakenvelopes at half intensity. The narrower the peak width, the faster thedesorption rate. The desorption rate can also be characterized by thedistance between the center of the tracer peak envelope and thedisappearance of an extract component which has just been desorbed. Thisdistance is again the volume of desorbent pumped during this timeinterval.

To further evaluate promising adsorbent systems and to translate thistype of data into a practical separation process requires actual testingof the best system in a continuous countercurrent liquid-solidcontacting device. The general operating principles of such a devicehave been previously described and are found in Broughton U.S. Pat. No.2,985,589. A specific laboratory size apparatus utilizing theseprinciples is described in deRosset et al., U.S. Pat. No. 3,706,812. Theequipment comprises multiple adsorbent beds with a number of accesslines attached to distributors within the beds and terminating at arotary distributing valve. At a given valve position, feed and desorbentare being introduced through two of the lines and the raffinate andextract streams are being withdrawn through two more. All remainingaccess lines are inactive and when the position of the distributingvalve is advanced by one index, all active positions will be advanced byone bed. This simulates a condition in which the adsorbent physicallymoves in a direction countercurrent to the liquid flow. Additionaldetails on the above-mentioned adsorbent testing apparatus and adsorbentevaluation techniques may be found in the paper "Separation of C₈Aromatics by Adsorption" by A. J. deRosset, R. W. Neuzil, D. J. Korous,and D. H. Rosback presented at the American Chemical Society, LosAngeles, Calif., Mar. 28 through Apr. 2, 1971.

Adsorbents to be used in the process of this invention will compriseweakly basic anion exchange resins possessing tertiary amine or pyridinefunctionality in a cross-linked polymeric matrix, e.g., acrylic orstyrene. They are especially suitable when produced in bead form, have ahigh degree of uniform polymeric porosity, exhibit chemical and physicalstability and good resistance to attrition (not common to macroreticularresins).

Adsorbents such as just described are manufactured by the Rohm and HaasCompany, and sold under the trade name "Amberlite." The types ofAmberlite polymers known to be effective for use by this invention arereferred to in Rohm and Haas Company literature as Amberlite adsorbentsXE-275 (IRA-35), IRA-68, and described in the literature as "insolublein all common solvents and having open structure for effectiveadsorption and desorption of large molecules without loss of capacity,due to organic fouling." Also, suitable are AG3-X4A and AG4-X4manufactured by Bio Rad and comparable resins sold by Dow Chemical Co.,such as Dowex 66, and Dow experimental resins made in accordance withU.S. Pat. Nos. 4,031,038 and 4,098,867.

The various types of polymeric adsorbents of these classes available,will differ somewhat in physical properties such as porosity volumepercent, skeletal density and nominal mesh sizes, and perhaps more so insurface areas, average pore diameter and dipole moment. The preferredadsorbents will have a surface area of 10-2000 square meters per gramand preferably from 100-1000 m² /g. Specific properties of the materialslisted above can be found in company literature and technical brochures,such as those in the following Table 1 which are incorporated herein byreference. Others of the general class are also available.

                  TABLE 1                                                         ______________________________________                                        Adsorbent                                                                              Matrix Type                                                                              Reference to Company Literature                           ______________________________________                                        AG3-4A   Polystyrene                                                                              Chromatography Electrophoresis                            (Bio Rad)           Immunochemistry Molecular                                                     Biology - HPLC - Price List M                                                 April 1987 (Bio-Rad)                                      AG4-X4   Acrylic    Chromatography Electrophoresis                                                Immunochemistry Molecular                                                     Biology - HPLC - Price List M                                                 April 1987 (Bio-Rad)                                      Dow      Polystyrene                                                                              U.S. Pat. Nos. 4,031,038 and                              Experimental        4,098,867                                                 Resins                                                                        Dowex 66 Polystyrene                                                                              Material Safety Data Sheet                                                    Printed 2/17/87                                                               (Dow Chemical USA)                                        IRA-35   Acrylic    Amberlite Ion Exchange Resins                             (XE-275)            (XE-275) Rohm & Haas Co. 1975                             IRA-68   Acrylic    Amberlite Ion Exchange Resins -                                               Amberlite IRA-68                                                              Rohm & Haas Co. April 1977                                ______________________________________                                    

Applications for Amberlite polymeric adsorbents suggested in the Rohmand Haas Company literature include decolorizing pulp mill bleachingeffluent, decolorizing dye wastes and removing pesticides from wasteeffluent. There is, of course, no hint in the literature of mysurprising discovery of the effectiveness of Amberlite polymericadsorbents in the separation of citric acid from Aspergillus-Nigerfermentation broths.

The adsorbent may be employed in the form of a dense compact fixed bedwhich is alternatively contacted with the feed mixture and desorbentmaterials. In the simplest embodiment of the invention, the adsorbent isemployed in the form of a single static bed in which case the process isonly semicontinuous. In another embodiment a set of two or more staticbeds may be employed in fixed bed contacting with appropriate valving sothat the feed mixture is passed through one or more adsorbent beds whilethe desorbent materials can be passed through one or more of the otherbeds in the set. The flow of feed mixture and desorbent materials may beeither up or down through the desorbent. Any of the conventionalapparatus employed in static bed fluid-solid contacting may be used.

Countercurrent moving bed or simulated moving bed countercurrent flowsystems, however, have a much greater separation efficiency than fixedadsorbent bed systems and are therefore preferred. In the moving bed orsimulated moving bed processes the adsorption and desorption operationsare continuously taking place which allows both continuous production ofan extract and a raffinate stream and the continual use of feed anddesorbent streams. One preferred embodiment of this process utilizeswhat is known in the art as the simulated moving bed countercurrent flowsystem. The operating principles and sequence of such a flow system aredescribed in U.S. Pat. No. 2,985,589 incorporated herein by referencethereto. In such a system it is the progressive movement of multipleliquid access points down an adsorbent chamber that simulates the upwardmovement of adsorbent contained in the chamber. Only four of the accesslines are active at any one time; the feed input stream, desorbent inletstream, raffinate outlet stream, and extract outlet stream access lines.Coincident with this simulated upward movement of the solid adsorbent isthe movement of the liquid occupying the void volume of the packed bedof adsorbent. So that countercurrent contact is maintained, a liquidflow down the adsorbent chamber may be provided by a pump. As an activeliquid access point moves through a cycle, that is, from the top of thechamber to the bottom, the chamber circulation pump moves throughdifferent zones which require different flow rates. A programmed flowcontroller may be provided to set and regulate these flow rates.

The active liquid access points effectively divided the adsorbentchamber into separate zones, each of which has a different function. Inthis embodiment of my process it is generally necessary that threeseparate operational zones be present in order for the process to takeplace although in some instances an optional fourth zone may be used.

The adsorption zone, zone 1, is defined as the adsorbent located betweenthe feed inlet stream and the raffinate outlet stream. In this zone, thefeedstock contacts the adsorbent, extract component is adsorbed, and araffinate stream is withdrawn. Since the general flow through zone 1 isfrom the feed stream which passes into the zone to the raffinate streamwhich passes out of the zone, the flow in this zone is considered to bea downstream direction when proceeding from the feed inlet to theraffinate outlet streams.

Immediately upstream with respect to fluid flow in zone 1 is thepurification zone, zone 2. The purification zone is defined as theadsorbent between the extract outlet stream and the feed inlet stream.The basic operations taking place in zone 2 are the displacement fromthe nonselective void volume of the adsorbent of any raffinate materialcarried into zone 2 by shifting of adsorbent into this zone and thedesorption of any raffinate material adsorbed within the selective porevolume of the adsorbent on the surfaces of the adsorbent particles.Purification is achieved by passing a portion of extract stream materialleaving zone 3 into zone 2 at zone 2's upstream boundary, the extractoutlet stream, to effect the displacement of raffinate material. Theflow of material in zone 2 is in a downstream direction from the extractoutlet stream to the feed inlet stream.

Immediately upstream of zone 2 with respect to the fluid flowing in zone2 is the desorption zone or zone 3. The desorption zone is defined asthe adsorbent between the desorbent inlet and the extract outlet stream.The function of the desorption zone is to allow a desorbent materialwhich passes into this zone to displace the extract component which wasadsorbed upon the adsorbent during a previous contact with feed in zone1 in a prior cycle of operation. The flow of fluid in zone 3 isessentially in the same direction as that of zones 1 and 2.

In some instances as optional buffer zone, zone 4, may be utilized. Thiszone, defined as the adsorbent between the raffinate outlet stream andthe desorbent inlet stream, if used, is located immediately upstreamwith respect to the fluid flow to zone 3. Zone 4 would be utilized toconserve the amount of desorbent utilized in the desorption step since aportion of the raffinate stream which is removed from zone 1 can bepassed into zone 4 to displace desorbent material present in that zoneout of that zone into the desorption zone. Zone 4 will contain enoughadsorbent so that raffinate material present in the raffinate streampassing out of zone 1 and into zone 4 can be prevented from passing intozone 3 thereby contaminating extract stream removed from zone 3. In theinstances which the fourth operational zone is not utilized theraffinate stream passed from zone 1 to zone 4 must be carefullymonitored in order that the flow directly from zone 1 to zone 3 can bestopped when there is an appreciable quantity of raffinate materialpresent in the raffinate stream passing from zone 1 into zone 3 so thatthe extract outlet stream is not contaminated.

A cyclic advancement of the input and output streams through the fixedbed of adsorbent can be accomplished by utilizing a manifold system inwhich the valves in the manifold are operated in a sequential manner toeffect the shifting of the input and output streams thereby allowing aflow of fluid with respect to solid adsorbent in a countercurrentmanner. Another mode of operation which can effect the countercurrentflow of solid adsorbent with respect to fluid involves the use of arotating disc valve in which the input and output streams are connectedto the valve and the lines through which feed input, extract output,desorbent input and raffinate output streams pass are advanced in thesame direction through the adsorbent bed. Both the manifold arrangementand disc valve are known in the art. Specifically rotary disc valveswhich can be utilized in this operation can be found in U.S. Pat. Nos.3,040,777 and 3,422,848. Both of the aforementioned patents disclose arotary type connection valve in which the suitable advancement of thevarious input and output streams from fixed sources can be achievedwithout difficulty.

In many instances, one operational zone will contain a much largerquantity of adsorbent than some other operational zone. For instance, insome operations the buffer zone can contain a minor amount of adsorbentas compared to the adsorbent required for the adsorption andpurification zones. It can also be seen that in instances in whichdesorbent is used which can easily desorb extract material from theadsorbent that a relatively small amount of adsorbent will be needed ina desorption zone as compared to the adsorbent needed in the buffer zoneor adsorption zone or purification zone or all of them. Since it is notrequired that the adsorbent be located in a single column, the use ofmultiple chambers or a series of columns is within the scope of theinvention.

It is not necessary that all of the input or output streams besimultaneously used, and in fact, in many instances one of the streamscan be shut off while others effect an input or output of material. Theapparatus which can be utilized to effect the process of this inventioncan also contain a series of individual beds connected by connectingconduits upon which are placed input or output taps to which the variousinput or output streams can be attached and alternately and periodicallyshifted to effect continuous operation. In some instances, theconnecting conduits can be connected to transfer taps which during thenormal operations do not function as a conduit through which materialpasses into or out of the process.

It is contemplated that at least a portion of the extract output streamwill pass into a separation means wherein at least a portion of thedesorbent material can be separated to produce an extract productcontaining a reduced concentration of desorbent material. Preferably,but not necessary to the operation of the process, at least a portion ofthe raffinate output stream will also be passed to a separation meanswherein at least a portion of the desorbent material can be separated toproduce a desorbent stream which can be reused in the process and araffinate product containing a reduced concentration of desorbentmaterial. The separation means will typically be a fractionation column,the design and operation of which is well-known to the separation art.

Reference can be made to D. B. Broughton U.S. Pat. No. 2,985,589, and toa paper entitled "Continuous Adsorptive Processing--A New SeparationTechnique" by D. B. Broughton presented at the 34th Annual Meeting ofthe Society of Chemical Engineers at Tokyo, Japan on Apr. 2, 1969, bothincorporated herein by reference, for further explanation of thesimulated moving bed countercurrent process flow scheme.

Although both liquid and vapor phase operations can be used in manyadsorptive separation process, liquid-phase operation is preferred forthis process because of the lower temperature requirements and becauseof the higher yields of extract product than can be obtained withliquid-phase operation over those obtained with vapor-phase operation.Adsorption conditions will include a temperature range of from about 20°C. to about 200° C. with about 50° C. to about 90° C. being morepreferred and a pressure range of from about atmospheric to about 500psig (3450 kPa gauge) being preferred to ensure liquid phase. Desorptionconditions will include the same range of temperatures and pressures asused for adsorption conditions.

The size of the units which can utilize the process of this inventioncan vary anywhere from those of pilot plant scale (see for example ourassignee's U.S. Pat. No. 3,706,812, incorporated herein by reference) tothose of commercial scale and can range in flow rates from as little asa few cc an hour up to many thousands of gallons per hour.

The following examples are presented to illustrate the selectivityrelationship that makes the process of my invention possible. Theexamples are not intended to unduly restrict the scope and spirit ofclaims attached hereto.

EXAMPLE I

In this example, four pulse tests were run with a weakly basic anionexchange resin having a tertiary amine function hydrogen bonded to asulfate ion, in a cross-linked gel-type acrylic resin matrix (AG4-X4made by Bio Rad Laboratories, Richmond, Calif.) having a tertiary aminefunction hydrogen bonded to a sulfate ion, in a cross-linked acrylicresin matrix to determine the ability of the adsorbent to separatecitric acid from its fermentation mixture of carbohydrates (DP1, DP2,DP3, including glucose, xylose, arabinose and raffinose) and ions ofsalts, including Na⁺, K⁺, Mg⁺⁺, Ca⁺⁺, Fe⁺⁺⁺, Cl⁻, SO₄.sup.═,PO₄.sup..tbd. and NO₃ ⁻, amino acids and proteins at a pH of 1.6. Thefirst test was run at a temperature of 75° C. The remaining tests wererun at 60° C. Citric acid was desorbed with water (FIG. 3) and sulfuricacid in two concentrations: 0.05 (FIG. 4) and 0.25N (FIG. 5). The fourthtest was like the second test (FIG. 4) except that it was made after theadsorbent was used with 24 bed volumes of feed. The fermentation feedmixture had the following composition:

                  TABLE 2                                                         ______________________________________                                        Feed Composition          Percent                                             ______________________________________                                        Citric Acid               40%                                                 Salts (K.sup.+, Na.sup.+, Ca.sup.++, Mg.sup.++, Fe.sup.+++)                                             1.5%                                                Carbohydrates (Sugars)    4%                                                  Others (SO.sub.4.sup.=, Cl.sup.-, PO.sub.4.sup.=, NO.sub.3.sup.-,                                       5%                                                  proteins and amino acids)                                                     Water                     49.5%                                               ______________________________________                                    

Retention volumes and separation factor (β) were obtained using thepulse test apparatus and procedure previously described. Specifically,the adsorbent was tested in a 70 cc straight column using the followingsequence of operations for the pulse test. Desorbent material wascontinuously run upwardly through the column containing the adsorbent ata nominal liquid hourly space velocity (LHSV) of about 1.0. A voidvolume was determined by observing the volume of desorbent required tofill the packed dry column. At a convenient time the flow of desorbentmaterial was stopped, and a 5 cc sample of feed mixture was injectedinto the column via a sample loop and the flow of desorbent material wasresumed. Samples of the effluent were automatically collected in anautomatic sample collector and later analyzed for salts and citric acidby chromatographic analysis. The extract and raffinate components werenot analyzed separately for the other feed components, e.g.,carbohydrates, proteins, etc. which were contained therein. From theanalysis of these samples, peak envelope concentrations were developedfor the feed mixture components. The retention volume for the citricacid was calculated by measuring the distance from the midpoint of thenet retention volume of the salt envelope as the reference point to themidpoint of the citric acid envelope. The separation factor, β, iscalculated from the ratio of the retention volumes of the twocomponents.

The results for these pulse test are shown in the following Table No. 3.

                  TABLE NO. 3                                                     ______________________________________                                                             Feed                                                     FIG. No.  Resin      Component  NRV   β                                  ______________________________________                                        3         AG4-X4     Salts 1    0.0   0.00                                                         Citric Acid                                                                              53.2  1.00                                                         Unknowns A 7.6   6.98                                                         Unknowns B 19.4  2.74                                                         Salts 2    53.0  1.00                                    4         AG4-X4     Salts      0.0   0.00                                                         Citric Acid                                                                              34.8  1.00                                                         Unknown A  -3.2  ****                                                         Unknown B  -0.5  ****                                    5         AG4-X4     Salts      0.0   0.00                                                         Citric Acid                                                                              24.6  1.00                                                         Unknowns A 30.9  0.79                                                         Unknowns B 37.5  0.65                                    6         AG4-X4     Salts      0.0   0.00                                                         Citric Acid                                                                              35.6  1.00                                                         Unknowns A 25.3  1.41                                                         Unknowns B 22.4  1.59                                    ______________________________________                                    

The results are also shown in FIGS. 3-5 in which it is clear that whilecitric acid is satisfactorily separated in the process, in highlypurified form, with water (FIG. 3), desorption by water is slower thandilute sulfuric acid as evidenced by larger net retention volume fordesorption of citric acid with water. After aging the adsorbent with 24bed volumes of feed, the adsorbent shows no signs of deactivation, asobserved in FIG. 6, which is substantially identical to FIG. 4(conducted under identical conditions with fresh adsorbent).

EXAMPLE II

The first pulse test of Example I was repeated using the same procedureand apparatus except that the temperature was 65° C. The desorbent waswater. This example presents the results of using a macroporous weaklybasic anionic exchange resin possessing a cross-linked polystyrenematrix (Dowex 66) with the same separation feed mixture as Example I(40% citric acid) in the first two pulse tests at a pH of 7.0 and 3.5(FIGS. 7A and 7B, respectively) to demonstrate the failure to accomplishthe desired separation when the pH is above the first ionizationconstant, pKa₁ =3.13, of citric acid, and more specifically in these twosamples, where the concentration of citric acid is 40%, when the pH isabove 1.7. In the third part of the example (represented by FIG. 7C),the feed was diluted to 13% citric acid and the pH reduced to 2.4. Whilethere is evident improvement, it is apparent that the pH and/orconcentration will have to be reduced further to prevent "breakout" ofthe citric acid. For example, at 13% concentration, it is estimated thatthe pH must be lowered to about 1.6 to 2.2.

FIGS. 7A and 7B are, respectively, graphical presentation of the resultsof the pulse test using Dowex 66 at pHs, respectively, of 7.0 and 3.5.FIGS. 7A and 7B show that citric acid "breaks through" with the salts(and carbohydrates) at the higher pHs. This problem can be partiallyalleviated by reducing the concentration to 13% and lowering the pH to2.4 as in FIG. 7C, where it is shown that only a small amount of citricacid is not adsorbed and "breaks through" in the raffinate while most isadsorbed onto the adsorbent resin (but not desorbed in this Figure).This separation, with adjustment of the concentration and pH to optimumlevels, clearly will have commercial utility.

EXAMPLE III

Three additional pulse tests under the same conditions as Example I,except as noted, were made on citric acid samples of the same feedcomposition, but with two different adsorbents. The desorbent in thefirst two samples was 0.05N H₂ SO₄ (FIGS. 8 and 9A) while water was usedin the third sample (FIG. 9B). The composition of the feed used was thesame as used in Example I. The temperature was 60° C. and the pH was1.6. The adsorbent #1 in the first test was a macroporous pyridinefunction-containing divinylbenzene cross-linked resin of the followingformula: ##STR3## where P is the polystyrene moiety forming the resin.The second adsorbent (#2), used in the second and third samples, is atertiary amine, also with a pyridine functional group, having thefollowing formula: ##STR4## where P is as defined above. Both resins arecross-linked with divinylbenzene. In some cases, while water is aneffective desorbent, with excellent separation, it is not strong enoughto recover the adsorbed citric acid quickly enough to make the processcommerically attractive. See FIG. 9B, in which the conditions are thesame as above, using adsorbent #2, where water is the desorbent. In thiscase citric acid does not elute until about 95 ml of desorbent havepassed through the adsorbent. Dilute sulfuric acid is, therefore, thepreferred desorbent, as will be apparent from the results shown in FIGS.8 and 9A. Also, from FIGS. 8, 9A and 9B, it will be seen that anexcellent separation of citric acid is obtained.

EXAMPLE IV

The procedure, conditions and apparatus previously described in ExampleI were used to separate four samples of citric acid from the same feedwith two different resins of the same class of adsorbent as Example I,(except that in the first and fourth samples, the column temperature was50° C. and the desorbent was 0.05N H₂ SO₄ ; in the second and thirdsamples the pH was 2.2 and the desorbent was dilute sulfuric acid at0.15N concentration). Both resins, IRA-68 and IRA-35, obtained from Rohmand Haas, have an amine function and the following structural formula:##STR5## where P is the polyacrylic matrix, and

    R' and R"═CH.sub.3.

Amberlite IRA-68 (FIGS. 10, 11A and 11B) is a gel-type resin. IRA-35(FIG. 12) is a macroreticular-type resin. The third sample (FIG. 11B)was identical to the second (FIG. 11A), except that the adsorbent hadpreviously been used to separate 69 bed columns of the feed. As seen inFIGS. 10 and 11A, both are excellent adsorbents for separating citricacid from its fermentation broth within the pH range of 1.6 to 2.2. FIG.11B, shown after aging the adsorbent with 69 bed volumes of feed,demonstrates the stability of the resin (little or no deactivation hastaken place) in this separation. Net retention volume (NRV) andselectivity (β) are shown in the following Table 4.

                  TABLE 4                                                         ______________________________________                                        FIG. No.                                                                              Resin       Component  NRV   β                                   ______________________________________                                        10      Amberlite   Salts      0.0   0.00                                             IRA-68      Citric Acid                                                                              39.9  1.00                                                         Unknown A  7.7   5.19                                                         Unknown B  9.7   4.11                                     11A     Amberlite   Salts      0.0   0.00                                             IRA-68      Citric Acid                                                                              26.7  1.00                                                         Unknown A  -2.3  ****                                                         Unknown B  4.2   6.36                                     11B     Amberlite   Salts      0.0   0.00                                             IRA-68      Citric Acid                                                                              26.5  1.004                                                        Unknown A  -2.85 -9.540                                                       Unknown B  4.2   6.27                                     12      Amberlite   Salts      0.0   0.00                                             IRA-35      Citric Acid                                                                              35.6  1.00                                                         Unknown A  -1.3  ****                                                         Unknown B  29.6  1.21                                     ______________________________________                                    

In a further comparison of this invention with the claimed adsorbents ofthe parent case, Ser. No. 943,219, several samples of the extract wereanalyzed for readily carbonizable impurities (RCS) (Food & ChemicalCodex (FCC) Monograph #3) and potassium level. RCS is determined in thefollowing manner: a 1 gm sample of the extract (actual concentration ofcitric acid is determined) is carbonized at 90° C. with 10 ml of 95% H₂SO₄. The carbonized substance is spectrophatometrically measured at 500nm using a 2-cm cell with a 0.5 inch diameter tube and the amount of RCSis calculated for 50% citric acid solution. The number arrived at can becompared with that obtained by using this procedure on the cobaltstandard solution of the FCC test mentioned above. Potassium isdetermined by atomic adsorption spectroscopy. For comparison, the sameanalytical determinations were made on a sample of the same feed and RCScalculated for 50% citric acid, but with XAD-4, an adsorbent of theclass claimed in the aforementioned parent case, Ser. No. 943,219. Theresults are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                        EXTRACT QUALITY (RCS/POTASSIUM) BY PULSE TEST                                                     RCS                                                                           Calculated        C.A. Net                                Adsorbent                                                                              Desorbent  (for 50% C.A.)                                                                            ppmK  Ret. Vol.                               ______________________________________                                        XAD-4    H.sub.2 O  6.86, 8.98  59, 137                                                                             13.0                                    AG4-X4   .05N.H.sub.2 SO.sub.4                                                                    1.77, 1.42  24, 81                                                                              34.8                                    #2 (Ex. III)                                                                           .05N.H.sub.2 SO.sub.4                                                                    3.17, 3.33  24, 54                                                                              30.8                                    #1 (Ex. III)                                                                           .05N.H.sub.2 SO.sub.4                                                                    2.17        62    31.0                                    ______________________________________                                    

An improvement in both reduction of levels of RCS and K for the weaklybasic resins of the invention is indicated by this data. In all samples,RCS was reduced by at least 50% and in two samples, K was reduced byover 50%. It is noted from the net retention volume that both classes ofadsorbents have good resolution, but the present adsorbents suffersomewhat from increased cycle times. The cycle times can be reduced byusing higher concentrations of sulfuric acid, e.g., up to about 0.2N, inthe preferred range of 0.1 to 0.2N.

In another embodiment, citric acid adsorbed on the adsorbent may beconverted in situ to a citrate before being desorbed, for example, byreaction with an alkaline earth metal of alkali metal hydroxide orammonium hydroxide and then immediately eluted using a metal hydroxide,ammonium hydroxide or water as the desorbent. Deactivation of theadsorbent by the unknown impurities may take place in time, but theadsorbent may be regenerated by flushing with a stronger desorbent,e.g., a higher concentration of sulfuric acid than the desorbent, analkali metal hydroxide or NH₄ OH, or an organic solvent, e.g., acetoneor alcohol.

What is claimed is:
 1. An adsorption process for separating citric acidfrom a fermentation broth feed mixture comprising contacting saidmixture with a water-insoluble, macroreticular or gel, weakly basicanionic exchange resin possessing tertiary amine or pyridine functionalgroups, said weakly basic anionic exchange resin having a crosslinkedacrylic or styrene resin matrix at adsorption conditions selected toselectively adsorb said citric acid, desorbing said citric acid with adesorbent comprising water or a dilute inorganic acid at desorptionconditions, said adsorption conditions including pH lower than the firstionization constant (pKa₁) of citric acid.
 2. The process of claim 1further characterized in that said adsorption and desorption conditionsinclude a temperature within the range of from about 20° C. to about200° C. and a pressure within the range of from about atmospheric toabout 500 psig (3450 kPa gauge).
 3. The process of claim 1 furthercharacterized in that the pH of said feed mixture is lower than thefirst ionization constant (PKa₁) of citric acid.
 4. The process of claim1 further characterized in that said adsorbent has a tertiary aminefunctional group and said matrix is a cross-linked acrylic resin.
 5. Theprocess of claim 1 further characterized in that said adsorbent has apyridine functional group and said matrix is a cross-linked polystyreneresin.
 6. The process of claim 5 further characterized in that saidadsorbent has a surface area of at least 10 m² /g.
 7. The process ofclaim 1 wherein said fermentation broth comprises citric acid,carbohydrates, protein, amino acid and salts.
 8. The process of claim 1wherein said desorbent is sulfuric acid at a concentration in the rangefrom about 0.01N to about 1N.
 9. A process for separating citric acidfrom a feed mixture comprising a fermentation broth, which processemploys a water-insoluble, macroreticular or gel, weakly basic anionicexchange resin adsorbent possessing tertiary amine functional groups orpyridine functional groups, said anionic exchange resin having acrosslinked acrylic or styrene resin matrix which process comprises thesteps of:(a) maintaining net fluid flow through a column of saidadsorbent in a single direction, which column contains at least threezones having separate operational functions occurring therein and beingserially interconnected with the terminal zones of said column connectedto provide a continuous connection of said zones; (b) maintaining anadsorption zone in said column, said zone defined by the adsorbentlocated between a feed input stream at an upstream boundary of said zoneand a raffinate output stream at a downstream boundary of said zone; (c)maintaining a purification zone immediately upstream from saidadsorption zone, said purification zone defined by the adsorbent locatedbetween an extract output stream at an upstream boundary of saidpurification zone and said feed input stream at a downstream boundary ofsaid purification zone; (d) maintaining a desorption zone immediatelyupstream from said purification zone, said desorption zone defined bythe adsorbent located between a desorbent input stream at an upstreamboundary of said zone and said extract output stream at a downstreamboundary of said zone; (e) passing said feed mixture into saidadsorption zone at adsorption conditions to effect the selectiveadsorption of said citric acid by said adsorbent in said adsorption zoneand withdrawing a raffinate output stream comprising the nonadsorbedcomponents of said fermentation broth from said adsorption zone; (f)passing a desorbent material comprising water or a dilute inorganic acidinto said desorption zone at desorption conditions to effect thedisplacement of said citric acid from the adsorbent in said desorptionzone; (g) withdrawing an extract output stream comprising said citricacid and desorbent material from said desorption zone; (h) passing atleast a portion of said extract output stream to a separation means andtherein separating at separation conditions at least a portion of saiddesorbent material; and, (i) periodically advancing through said columnof adsorbent in a downstream direction with respect to fluid flow insaid adsorption zone the feed input stream, raffinate output stream,desorbent input stream, and extract output stream to effect the shiftingof zones through sid adsorbent and the production of extract output andraffinate output streams.
 10. The process of claim 9 furthercharacterized in that it includes the step of passing at least a portionof said extract output stream to a separation means and thereinseparating at separation conditions at least a portion of said desorbentmaterial to produce an extract product having a reduced concentration ofdesorbent material.
 11. The process of claim 9 further characterized inthat it includes the step of maintaining a buffer zone immediatelyupstream from said desorption zone, said buffer zone defined as theadsorbent located between the desorbent input stream at a downstreamboundary of said buffer zone and the raffinate output stream at anupstream boundary of said buffer zone.
 12. The process of claim 9further characterized in that said adsorption conditions and desorptionconditions include a temperature within the range of from about 20° C.to about 200° C. and a pressure within the range of from aboutatmospheric to about 500 psig (3450 kPa gauge) to ensure liquid phase.13. The process of claim 9 wherein said desorbent is sulfuric acid at aconcentration in the range from about 0.01N to about 1N.
 14. The processof claim 9 further characterized in that the pH of said feed mixture islower than the first ionization constant (pKa₁) of citric acid.
 15. Theprocess of claim 9 wherein the pH of the said feed mixture is below3.13.
 16. The process of claim 9 wherein said fermentation brothcomprises citric acid, carbohydrates and salts.
 17. The process of claim16 wherein said fermentation broth additionally contains proteins andamino acids.
 18. The process of claim 1 wherein said desorbent is adilute sulfuric acid.
 19. The process of claim 18 wherein theconcentration of said sulfuric acid is from 0.01N to 1.0N.
 20. Theprocess of claim 18 wherein the concentration of said sulfuric acid isfrom 0.05N to 0.25N.
 21. The process of claim 1 wherein said adsorbentis in the sulfate form.